1. Field of the Invention
This invention relates to a silicon nitride powder having a high packing density and consisting of the majority of isometric particles and a minimized content of coarse particles so that a high density compact can be molded from the powder and a high strength body can be sintered from the compact, and a method for preparing the same.
2. Prior Art
Recently, silicon nitride draws attention because of its heat resistance, strength, and corrosion resistance. End products were generally produced by adding an organic binder to silicon nitride powder and oxide co-agent in powder form and dispersing the mix in a suitable medium, typically water to form a slurry. Compacts of desired shape are formed from the slurry either by granulating the slurry and compression molding the granules or by directly casting the slurry in a mold. The compacts are heated for burning out the binder and then sintered.
It is known that the slurry formed midway the process should have a high concentration and a low viscosity in order to obtain end products having satisfactory physical properties. With respect to the shaping stage prior to sintering, if the molded compact were increased in density, it would have increased density and reduced shrinkage upon sintering, which is advantageous in the strength and dimensional precision of sintered bodies.
Silicon nitride powder is conventionally prepared by silica reduction, imide decomposition or direct nitriding of metallic silicon powder. It was difficult to prepare a high concentration, low viscosity slurry from the conventional silicon nitride powder using water as the medium. Then in molding compacts, it was difficult to increase the density of compacts. Further the presence of coarse particles prevented the compacts from being sintered to high strength.